The rapid rise in frequency of melanic morphs in several moth species, espe
cially the peppered moth Biston betularia, in industrial regions during the
19th century, and the subsequent rapid decline, indicate the action of str
ong selection. There has recently been a tendency to criticise and question
all aspects of research on industrial melanism, including the experiments
which suggest that selective predation plays an important part in the chang
es. These experiments are reexamined, together with evidence for changes in
appearance of tree surfaces and for relation of initial melanic frequency
to subsequent rate of decline. It is suggested that intense pollution may h
ave been required to drive the carbonaria morph to a high frequency, with f
requency patterns over a mosaic environment smoothed by migration. Improvem
ents in these extreme locations then triggered the decline, with little ind
ication of the environmental changes in areas of moderate pollution. Reason
s for criticism of past work are discussed. Industrial melanism continues t
o provide an exceptional opportunity to analyse a pattern of selection and
change in gene frequency. (C) 2000 The Linnean Society of London.